Process for preparing emulsions that are polymerizable to absorbent foam materials

ABSTRACT

Disclosed is a process for the continuous preparation of high internal phase emulsions which are suitable for subsequent polymerization into polymeric foam materials that, upon dewatering, act as absorbents for aqueous body fluids. The process involves continuous introduction of a certain type of monomer-containing oil phase and a certain type of electrolyte-containing water phase into a dynamic mixing zone at relatively low water to oil phase ratios. Flow rates are then steadily adjusted to increase the water to oil ratio of the streams fed to the dynamic mixing zone while subjecting the dynamic mixing zone contents to shear agitation which is sufficient to thereby form a high internal phase emulsion that, upon subsequent polymerization, provides a foam having an average cell size of from about 5 to 100 microns. The formation of such a stable high internal phase emulsion is completed by feeding the contents of the dynamic mixing zone to and through a static mixing zone.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of the copending applicationhaving U.S. Ser. No. 07/743,947, filed Aug. 12, 1991 in the names ofThomas A. DesMarais, Stephen T. Dick and Thomas M. Shiveley.

FIELD OF THE INVENTION

This invention relates to a continuous process for preparing certainhigh internal phase water-in-oil emulsions. Such emulsions containparticular types of monomeric materials dissolved in the oil phase ofthe emulsion such that, when the emulsions are subjected topolymerization conditions, especially useful polymeric foam structuresare realized. The specific emulsions which are prepared by the processherein are those which, when polymerized, provide foam structures thatfind particular utility for absorbing aqueous body fluids. These foamsare thus suitable for use in absorbent products such as diapers andother incontinence management products.

BACKGROUND OF THE INVENTION

Water-in-oil emulsions having a relatively high ratio of water phase tooil phase are known in the art as high internal phase emulsions ("HIPEs"or "HIPE" emulsions). Continuous processes for preparing HIPE emulsionsare disclosed, for example, in Lissant; U.S. Pat. No. 3,565,817; IssuedFeb. 23, 1971 and Bradley et al; British Patent Application 2194166A;Published Mar. 2, 1988.

HIPE emulsions which contain polymerizable comonomers in their externaloil phase have been made and polymerized in order to study the geometricconfiguration of the oil and water phases of such emulsions. Forexample, Lissant and Mahan, "A Study of Medium and High Internal PhaseRatio Waver/Polymer Emulsions," Journal of Colloid and InterfaceScience, Vol. 42, No. 1, January, 1973, pp. 201-208 discloses thepreparation of water-in-oil emulsions which contain 90% internal waterphase and which utilize styrene monomer in the oil phase. Such emulsionsare prepared by subjecting the combined oil and water phases toagitation using an emulsifier and are subsequently polymerized to form arigid porous structures having a cellular configuration determined bythe phase relationship of its emulsion precursor.

Preparation of HIPE emulsions suitable for polymerization to porousstructures, e.g., foams, useful for carrying and/or absorbing liquidsare also known. For example, Barby et al, U.S. Pat. No. 4,797,310,Issued Jan. 10, 1989; Jones et al, U.S. Pat. No. 4,612,334; Iissued Sep.16, 1986; Haq et al, U.S. Pat. No. 4,606,958, Issued Apr. 19, 1986; andBarby et al, U.S. Pat. No. 4,533,953, Issued Jun. 11, 1989 all discloseporous polymeric materials which can be prepared from HIPE emulsions andwhich are useful for delivering liquids such as cleaning solutions tohard surfaces via products such as wipers and cleaning cloths.

The prior art has also recognized that the nature and characteristics ofthe porous polymeric foam materials formed by polymerizing HIPEemulsions is very much dependent on both the type of components whichmakeup the polymerizable HIPE emulsion and the process conditions usedto form the emulsion. For example, Unilever, European Patent ApplicationNo. 60138, Published Sep. 15, 1982 discloses a process for preparingabsorbent porous polymers (i.e., foams) from high internal phaseemulsions comprising at least 90% by weight of water with the oil phasecontaining polymerizable monomers, surfactant and a polymerizationcatalyst. Edwards et al, U.S. Pat. No. 4,788,225, Issued Nov. 29, 1988discloses the preparation of porous polymer materials which are renderedelastic by selecting certain monomer types (styrene,alkyl(meth)acrylates, crosslinker) and by using certain processingconditions to control the cell size of the eventually resulting porouspolymer. Unilever, European Patent Application EP-A-299,762, PublishedJan. 18, 1989 discloses that the use of an electrolyte in the waterphase of polymerizable HIPE emulsions can affect the size of theopenings between cells of the eventually resulting porous polymeric foammaterial.

Notwithstanding the fact that the existence and synthesis ofpolymerizable HIPE emulsions is known in the art, preparation of HIPEemulsions suitable for polymerization to useful absorbent foam materialis not without its difficulties. Such HIPE emulsions, and especiallyHIPE emulsions having a very high ratio of water phase to oil phase,tend to be unstable. Very slight variations in monomer/crosslinkercontent in the emulsion, emulsifier selection, emulsion componentconcentrations, and temperature and/or agitation conditions can causesuch emulsions to "break" or to separate to at least some degree intotheir distinct water and oil phases. Even if stable emulsions can berealized, alterations in emulsion composition and processing conditionscan significantly affect the properties and characteristics of theeventually realized polymeric foam materials, thereby rendering suchfoam materials either more or less useful for their intended purpose.Such HIPE emulsion preparation difficulties can become even moretroublesome when there is a need to produce polymerizable emulsions viaa continuous process on an industrial or pilot plant scale in order toprovide commercially useful or developmental quantities of polymericabsorbent foam materials.

Given the foregoing considerations, it is an object of the presentinvention to provide a process for preparing certain types of highinternal phase emulsions that can be polymerized to form foam materialsespecially useful as an absorbent for aqueous body fluids, i.e., foamswhich are useful in absorbent product such as diapers.

It is a further object of the present invention to provide such a HIPEemulsion preparation process which can be carried out on a continuousbasis.

It is a further object of the present invention to provide such acontinuous HIPE emulsion preparation process which can be operated on acommercially meaningful scale.

SUMMARY OF THE INVENTION

The present invention provides a continuous process for the preparationof certain types of high internal phase emulsions that are themselvessuitable for subsequent polymerization into absorbent foam materials.This process comprises the steps of:

Providing separate water phase and oil phase liquid feed streams ashereinafter defined;

Simultaneously introducing these liquid feed streams into a dynamicmixing zone at flow rate such that the water to oil weight ratio ofliquid introduced ranges from about 2:1 to 10:1;

Subjecting the combined water and oil phase feed streams to sufficientshear agitation in the dynamic mixing zone to at least partially form anemulsified mixture therein while maintaining steady, non-pulsating flowrates for the oil and water phase streams;

Steadily increasing the water to oil weight ratio of the feed streamsfed to the dynamic mixing zone to a value of from about 12:1 to 100:1 ata rate of increase that does not break the emulsion in the dynamicmixing zone, while maintaining certain conditions in the dynamic mixingzone as hereinafter described;

Continuously withdrawing emulsified contents of the dynamic mixing zoneand continuously feeding these contents into a static mixing zonewherein they are subjected to additional shear agitation suitable forforming a stable high internal phase emulsion having a water-to-oilratio of from about 12:1 to 100:1; and

Continuous withdrawing the stable high internal phase emulsion from thestatic mixing zone so that it can be polymerized into a solid absorbentfoam material.

In such a process, the liquid feed stream of the oil phase comprisesfrom about 3 to 41 weight percent of a substantially water-insoluble,monofunctional glassy monomer component; from about 27 to 73 weightpercent of a substantially water-insoluble, monofunctional rubberycomonomer component; from about 8 to 30 weight percent of asubstantially water-insoluble, polyfunctional cross-linking agentcomponent and from about 2 to 33 weight percent of an emulsifiercomponent which is soluble in the oil phase and which is suitable forforming a stable water-in-oil emulsion. The liquid feed stream of thewater phase comprises an aqueous solution containing from about 0.2% to40% by weight of a water-soluble electrolyte.

As and after the water to oil weight ratio is increased by altering therates at which the feed streams are introduced into the dynamic mixingzone, the emulsified contents of the dynamic mixing zone are maintainedat a temperature of from about 25° C. to 70° C. Furthermore, theemulsified contents of the dynamic mixing zone are also subjected tocontinued shear agitation which is sufficient to eventually form a highinternal phase emulsion that, upon subsequent polymerization, provides afoam material having an average cell size of from about 5 to 100microns. The absorbent foams formed by polymerizing the emulsionprepared by the process herein will have these average cell sizecharacteristics and will be especially suitable for use in absorbingaqueous body fluids.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 of the drawings is a photomicrograph of the interstices of apolymerized HIPE emulsion of the type produced by the process of thepresent invention.

FIG. 2 of the drawings is a schematic flow diagram showing anarrangement of apparatus and equipment which can be used for carryingout the process of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The emulsions which are prepared via the process of the presentinvention are water-in-oil emulsions having therein a relatively highratio of water phase to oil phase. As indicated hereinbefore, emulsionsof this type which have these relatively high water to oil phase ratiosare known in the art as high internal phase emulsions (i.e., "HIPEs" or"HIPE" emulsions). HIPE emulsions having the oil and water phasecharacteristics of the present invention are suitable for polymerization(and dewatering) into foam materials which are especially useful asabsorbents for aqueous body fluids.

Each of the essential steps used to prepare HIPE emulsions of this typevia a continuous process is described in detail as follows:

A) Provision of the Oil Phase Feed Stream

The particular oil phase incorporated into the HIPE emulsions preparedby the process of this invention comprises monomers which polymerize toform a solid foam structure when the emulsions made from such an oilphase are eventually subjected to polymerization conditions. Themonomers essentially utilized in this oil phase include a principalmonomer component, a comonomer component and a cross-linking agentcomponent. Selection of particular types and amounts of monofunctionalprincipal monomer(s) and comonomer(s) and polyfunctional cross-linkingagent(s) can be important to the realization of absorbent HIPE-basedfoam materials having the desired combination of properties which rendersuch foam materials suitable for use as absorbents for body fluids.

The principal monofunctional monomer component utilized in the oil phaseused to prepare the HIPE emulsions herein comprises one or more monomersthat tend to impart glass-like properties to the eventually resultingfoam structure. Such monomers are hereinafter referred to as "glassy"monomers, and are, for purposes of this invention, defined as monomericmaterials which would produce high molecular weight (greater than 6000)homopolymers having a glass transition temperature, T_(g), above about40° C. The preferred monofunctional glassy monomer type is astyrene-based monomer with styrene itself being the most preferredmonomer of this kind. Substituted, e.g., monosubstituted, styrene suchas p-methylstyrene may also be employed. The monofunctional glassymonomer component will normally comprise from about 3% to 41%, morepreferably from about 7% to 40% by weight of the oil phase used to formthe HIPE emulsions herein.

Along with the principal glassy monomer material, a monofunctionalcomonomer component will also be present in the oil phase used in theinstant HIPE emulsion preparation process. Such a monofunctionalcomonomer component comprises one or more comonomers which tend toimpart rubber-like properties to the foams which eventually result fromthe polymerization of the emulsions prepared herein. Such comonomers arehereinafter referred to as "rubery" comonomers and are, for purposes ofthis invention, defined as monomeric materials which would produce highmolecular weight (greater than 10,000) homopolymers having a glasstransition temperature, T_(g), of about 40° C. or lower. Monofunctionalrubbery comonomers of this type include, for example, alkylacrylates,alkylmethacrylates, allylacrylate, butadiene, substituted butadienes,vinylidine halides and combinations of such comonomers and comonomertypes. Preferred rubbery comonomers include butylacrylate,2-ethylhexylacrylate, butadiene, isoprene and combinations of thesecomonomers. Of all of these species, butylacrylate and2-ethylhexylacrylate are the most preferred. The monofunctional rubberycomonomer component will generally comprise from about 27% to 73%, morepreferably from about 27% to 66%, by weight of the oil phase used toform the HIPE emulsions herein.

Within the oil phase used to prepare the HIPE emulsions herein, both themonofunctional glassy principal monomer(s) and the monofunctionalrubbery comonomer(s) must be present within the hereinbefore recitedconcentration ranges. In addition, the molar ratio of monofunctionalglassy monomer component to the monofunctional rubbery component in theoil phase will generally range from about 1:25 to 1.5:1, more preferablyfrom about 1:9 to 1.5:1.

Since the polymer chains formed from the glassy monomer(s) and therubbery comonomer(s) are to be cross-linked when the emulsions preparedherein are subsequently polymerized, the oil phase must also contain apolyfunctional cross-linking agent. As with the monofunctional monomersand comonomers, selection of a particular type and amount ofcross-linking agent can be very important to the eventual realization ofHIPE emulsions which are polymerizable to foams having the desiredcombination of structural, mechanical, and fluid-absorbing properties.

Depending upon the type and amounts of monofunctional monomers andcomonomers utilized, and depending further upon the desiredcharacteristics of the eventually realized polymeric foams, thepolyfunctional cross-linking agent component for use in the oil phasecan be selected from a wide variety of polyfunctional, preferablydifunctional, monomers. Thus, the cross-linking agent may be a divinylaromatic material such as divinylbenzene, divinyltolulene ordiallylphthalate. Alternatively, divinyl aliphatic cross-linkers such asany of the diacrylic acid esters of polyols can be utilized. Thecross-linking agent found to be suitable for preparing the mostacceptable foam-forming HIPE emulsions herein is divinylbenzene.

The cross-linking agent of whatever type will generally be employed inthe oil phase used in the emulsion-forming process herein in an amountof from about 8% to 40%, more preferably from about 10% to 25%, byweight. Amounts of cross-linking agent(s) within such ranges willgenerally provides a cross-linker molar concentration of from about 5mole percent to about 60 mole percent, based on total monomers presentin the oil phase.

The major portion of the oil phase used to prepare the HIPE emulsionsherein will comprise the aforementioned monomers, comonomers andcross-linking agents which eventually form the polymeric foamabsorbents. It is therefore essential that these monomers, comonomersand cross-linking agents be substantially water-insoluble so that theyare primarily soluble in the oil phase and not the water phase of theemulsions herein. Use of such substantially water-insoluble monomermaterials ensures that HIPE emulsions of appropriate characteristics andstability will be realized.

It is, of course, preferred that the monomers, comonomers andcross-linking agents used to form the foam precursor emulsions herein beof the type such that the eventually formed foam polymer is suitablynon-toxic and sufficiently chemically stable. Thus such monomers,comonomers and cross-linking agent should preferably have little or notoxicity in the very low residual concentrations wherein they may beencountered during post-polymerization foam processing and/or use.

Another essential component of the oil phase used to form the HIPEemulsions in accordance with the present invention comprises anemulsifier which permits formation of stable HIPE emulsions. Suchemulsifiers are those which are soluble in the oil phase used to formthe emulsion. Emulsifiers utilized may be nonionic, cationic, anionic oramphoteric provided the emulsifier or combination of emulsifiers willform a stable HIPE emulsion. Preferred types of emulsifiers which can beused to provide an emulsifier component having suitable characteristicsinclude the sorbitan fatty acid esters, polyglycerol fatty acid esters,polyoxyethylene (POE) fatty acids and esters. Especially preferred arethe sorbitan fatty acid esters such as sorbitan monolaurate (SPAN® 20),sorbitan monooleate (SPAN® 80) and combinations of sorbitan trioleate(SPAN® 85) and sorbitan monooleate (SPAN® 80). One such particularlypreferred emulsifier combination comprises the combination of sorbitanmonooleate and sorbitan trioleate in a weight ratio greater than orequal to about 3.1, more preferably about 4:1. Other operableemulsifiers include TRIODAN® 20 which is a commercially availablepolyglycerol ester marked by Grindsted and EMSORB 2502 which is asorbitan sesquioleate marketed by Henkel.

The emulsifier component will generally comprise from about 2% to 33% byweight of the oil phase used to form the HIPE emulsions herein which inturn are used to prepare polymeric absorbent foams. More preferably, theemulsifier component will comprise from about 4% to 25% by weight of theoil phase.

In addition to the monomeric and emulsifier components hereinbeforedescribed, the oil phase used to form polymerizable HIPE emulsionsherein may also contain additional optional components. One suchoptional oil phase component may be an oil soluble polymerizationinitiator of the general type hereinafter described. Another possibleoptional component of the oil phase may be a substantially waterinsoluble solvent or carrier for the oil phase monomer, cross-linkerand/or emulsifier components. A solvent or carrier of this type must, ofcourse, not be capable of dissolving the eventually polymerizedmonomers. Use of such a solvent is not preferred, but if such a solventor carrier is employed, it will generally comprise no more than about10% by weight of the oil phase.

The oil phase, as hereinbefore described, may itself be prepared in anysuitable manner by combining the essential and optional components usingconventional techniques. Such a combination of components may be carriedout in either continuous or batch-wise fashion using any appropriateorder of component addition. The oil phase so prepared will generally beformed and stored in a feed tank, from which tank the oil phase can beprovided in a liquid feed stream of any desired flow rate as hereinafterdescribed.

B) Provision of the Water Phase Feed Stream

As indicated, an oil phase as hereinbefore described is the continuousexternal phase in the HIPE emulsions to be polymerized to realizeabsorbent foams. The discontinuous internal phase of such polymerizableHIPE emulsions is a water phase which will generally be an aqueoussolution containing one or more dissolved components. Like the oilphase, the water phase used to form the HIPE emulsions herein will befed to the process as a separate stream.

One essential dissolved component of the water phase is a water-solubleelectrolyte. The dissolved electrolyte in the water phase used to formthe HIPE emulsions herein serves to minimize the tendency of monomersand crosslinkers which are primarily oil soluble to also dissolve in thewater phase. This, in turn, can minimize the extent to which, duringsubsequent polymerization of the emulsion, polymeric material fills thecell windows at the oil/water intefaces formed by the water phasebubbles. Thus the presence of electrolyte and the resulting ionicstrength of the water phase can determine whether and to what degree theeventually resulting polymeric foams may be open-celled.

Any electrolyte which provides ionic species to impart ionic strength tothe water phase may be used. Preferred electrolytes are mono-, di-, ortri-valent inorganics salts such as the water-soluble halides (e.g.,chlorides), nitrates and sulfates of alkali metals and alkaline earthmetals. Examples include sodium chloride, calcium chloride, sodiumsulfate and magnesium sulfate. Calcium chloride is the most preferredelectrolyte for use in the water phase.

Generally electrolyte will be present in the water phase used herein toform the HIPE emulsions in a concentration which ranges from about 0.2%to about 40% by weight of the water phase. More preferably, theelectrolyte will comprise from about 0.5% to 20% by weigh of the waterphase.

The HIPE emulsions formed via the process herein will, in addition tothe essential oil and water phase components hereinbefore described,also typically contain a polymerization initiator. Such an initiatorcomponent will generally be added to the water phase used to form theHIPE emulsions and can be any conventional water-soluble free radicalinitiator. Materials of this type include peroxygen compounds such assodium, potassium and ammonium persulfates, caprylyl peroxide, benzoylperoxide, hydrogen peroxide, cumene hydroperoxides, tertiary butyldiperphthalate, tertiary butyl perbenzoate, sodium peracetate, sodiumpercarbonate and the like. Conventional redox initiator systems can alsobe utilized. Such systems are formed by combining the foregoingperoxygen compounds with reducing agents such as sodium bisulfite,L-ascorbic acid or ferrous salts.

The initiator material can comprise up to about 5 mole percent based onthe total moles of polymerizable monomers present in the oil phase. Morepreferably, the initiator comprises from about 0.001 to 0.5 mole percentbased on the total moles of polymerizable monomers (i.e., monomers,comonomers, cross-linkers) in the oil phase. When used in thewater-phase, such initiator concentrations can be realized by addinginitiator to the water phase to the extent of from about 0.02% to 0.4%,more preferably from about 0.1% to 0.2% by weight of the water phase.

As with the oil phase, the water phase, containing the essential andoptional components hereinbefore described, may itself be prepared bycombining these components in conventional manner. Thus, the water phasemay be prepared in either continuous or batch-wise fashion using anyappropriate order of addition of water phase components. As with the oilphase, the water phase will generally be prepared and stored in aseparate feed tank which is equipped with means for delivering a waterphase liquid stream from this tank at any desired flow rate.

C) Initial Introduction of Oil and Water Phase Feed Streams Into theDynamic Mixing Zone

The liquid streams of both oil and water phases as hereinbeforedescribed are initially combined by simultaneously introducing liquidfeed streams of both these phases together into a dynamic mixing zone.This dynamic mixing zone, and the emulsion-forming agitation imparted tothe liquid contents thereof, are hereinafter described in greaterdetail.

During this stage of initial combination of liquid feed streams of theoil and water phases, flow rates of these feed streams are set so thatthe weight ratio of water phase to oil phase being introduced into thedynamic mixing zone is well below that of the HIPE emulsions which areto be eventually realized. In particular, flow rates of the oil andwater phase liquid streams are set such that the water to oil weightratio during this initial introduction stage ranges from about 2:1 to10:1, more preferably from about 2.5:1 to 5:1. The purpose of combiningthe oil and water phase streams at these relatively low water to oilratios is to permit formation in the dynamic mixing zone of at leastsome amount of a water-in-oil emulsion which is relatively stable anddoes not readily "break" under conditions encountered in the dynamicmixing zone.

The actual flow rates of the oil and water phase liquid feed streamsduring this stage of the initial introduction into the dynamic mixingzone will vary depending upon the scale of the emulsion preparationprocess desired. For pilot plant scale operations, the oil phase flowrate during this initial introduction stage can range from about 0.02 to0.2 liter/minute, and the water phase flow rate can range from about0.04 to 2.0 liters/minute. For commercial scale operations, the oilphase flow rate during this initial introduction stage can range fromabout 10 to 25 liters/minute, and the water phase flow rate can rangefrom about 20 to 250 liters/minute.

For the initial startup of the process herein, the dynamic mixing zoneis filled with oil and water phase liquid before agitation begins.During this filling stage, gas is vented from the dynamic mixing zone.Before agitation begins, the liquid in the dynamic mixing zone is in twoseparate phases, i.e., an oil phase and a water phase. Once the dynamicmixing zone is filled with liquid, agitation is begun as described ingreater detail hereinafter, and emulsion begins to form in the dynamicmixing zone. At this point, oil and water phase flow rates should be setwithin the ranges hereinbefore set forth to provide the relatively lowinitial water to oil weight ratio within the range which is alsohereinbefore set forth.

D) Initial Emulsion Formation in the Dynamic Mixing Zone

As noted the oil and water phase feed streams are initially combined bymeans of their simultaneous introduction into a dynamic mixing zone. Forpurposes of the present invention, the dynamic mixing zone comprises acontainment vessel for liquid components, which vessel is equipped withmeans for imparting shear agitation to the liquid contents of thevessel. Such shear agitation means must be suitable for providingagitation or mixing beyond that which arises by virtue of simple flow ofliquid material through the containment vessel.

The agitation means which is employed as an element of the dynamicmixing zone can comprise any conventional apparatus which imparts therequisite amount of shear agitation to the liquid contents of thedynamic mixing zone. One suitable type of agitation apparatus is a pinimpeller which comprises a cylindrical shaft from which a number of rows(flights) of cylindrical pins extend radially. The number, dimensions,and configuration of the pins on the impeller shaft can vary widely,depending upon the amount of shear agitation which the impeller is toimpart to the dynamic mixing zone liquid contents. As discussed morefully hereinafter with respect to the drawing, a pin impeller of thistype can be mounted within a generally cylindrical mixing vessel whichserves as the dynamic mixing zone. The impeller shaft is positionedgenerally parallel to the direction of liquid flow through thecylindrical dynamic mixing zone. Shear agitation is provided by rotatingthe impeller shaft at a speed which imparts the requisite degree ofshear agitation to the liquid material passing through the dynamicmixing zone.

The shear agitation imparted to the liquid contents which have beeninitially introduced into the dynamic mixing zone is that which issufficient to form at least some of the dynamic mixing zone contentsinto a water-in-oil emulsion having water to oil ratios within theranges set forth hereinbefore. Frequency such shear agitation at thispoint will range from about 1000 to 7000 sec.⁻¹, more preferably, fromabout 1500 to 3000 sec.⁻¹. The amount of shear agitation need not beconstant but may be varied over the time needed to effect such emulsionformation. As indicated, not all of the water and oil phase materialwhich has been introduced into the dynamic mixing zone at this pointneed be incorporated into the water-in-oil emulsion so long as at leastsome emulsion of this type (e.g., the emulsion comprises at least about90% by weight of the liquid effluent from the dynamic mixing zone) isformed in and flows through the dynamic mixing zone.

Once agitation begins, it is important that both the oil and water phaseflow rates be steady and non-pulsating. This is because sudden orprecipitous changes in these flow rates can cause the emulsion that hasbeen formed in the dynamic mixing zone to break. Accordingly, changes inflow rates of the oil and/or water phases should be gradual enough toprevent any significant separation of the emulsion which has formed inthe dynamic mixing zone into non-emulsified oil and water phases.

E) HIPE Formation in the Dynamic Mixing Zone

After a water-in-oil emulsion of relatively low water-to-oil ratio hasbeen formed in the dynamic mixing zone by selection of suitable flowrates and shear agitation conditions as hereinbefore described, stepsare taken to convert the emulsion so formed, along with the additionalnon-emulsified contents of the dynamic mixing zone, into a high internalphase emulsion, e.g., a HIPE. This is accomplished by altering therelative flow rates of the water and oil phase streams being fed intothe dynamic mixing zone. Thus, such an increase in water-to-oil ratio ofthe phases being introduced can be accomplished by increasing the waterphase flow rate, by decreasing the oil phase flow rate or by acombination of these steps. The water-to-oil ratios to be eventuallyrealized by such an adjustment of the water phase and/or oil phase flowrates will generally range from about 12:1 to 100:1, more preferablyfrom about 20:1 to 70:1, most preferably from about 25:1 to 50:1.

Adjustment of the oil phase and/or water phase flow rates to increasethe water to oil phase ratio of the liquid material being fed to thedynamic mixing zone can begin immediately after initial formation ofemulsion in the dynamic mixing zone. This will generally occur soonafter agitation is begun in the dynamic mixing zone. The length of timetaken to increase the water to oil ratio to the ultimately desiredhigher ratio will depend on the scale of the process being carried outand the magnitude of the eventual water to oil ratio to be reached.Frequently the duration of the flow rate adjustment period needed toincrease water to oil ratios will range rom about 1 to 5 minutes.

In altering the relative water and oil phase flow rates to achieve theultimately desired relatively high water-to-oil ratios for HIPE emulsionpreparation, care should be taken to approach these desired ratiosgradually. As noted hereinbefore, sudden or precipitous changes in thewater phase and/or oil phase flow rates can cause the emulsion in thedynamic mixing zone to "break" before or shortly after the requisiteHIPE emulsions can be formed. The actual rate of increase of thewater-to-oil ratio in the streams being fed to the dynamic mixing zonewill be dependent upon the particular components of the emulsion beingprepared as well as the scale of the process being carried out. For anygiven HIPE formula and process setup, emulsion stability can becontrolled by simply monitoring the nature of the effluent from theprocess to ensure that it comprises at least some material (e.g., atleast 90% of the total effluent) in substantially HIPE emulsion form.

Conditions within the dynamic mixing zone during emulsion formation canalso affect the nature of the HIPE emulsion (and ultimately thepolymerized absorbent foam made therefrom) which is prepared by theprocess herein. One element of dynamic mixing zone conditions which canimpact on the character of HIPE produced is the temperature of theemulsion components within the dynamic mixing zone. Generally theemulsified contents of the dynamic mixing zone should be maintained at atemperature of from about 25° to 70° C., more preferably from about 35°to 65° C., during the HIPE formation step.

Another element of the dynamic mixing zone conditions involves theamount of shear agitation imparted to the contents of the dynamic mixingzone both during and after adjustment of the water and oil phase flowrates to form the HIPE-ratio emulsions herein. The amount of shearagitation imparted to the emulsified material in the dynamic mixing zonewill directly impact on the size of the cells which make up theeventually-realized absorbent foam material. For a given set of emulsioncomponent types and ratios, and for a given combination of flow ratesinto the dynamic mixing zone, subjecting the dynamic mixing zone liquidcontents to greater amounts of shear agitation will tend to reduce thesize of the cells in the absorbent foam material produced from the HIPEemulsion eventually realized herein.

Foam cells, and especially cells which are formed by polymerizing amonomer-containing oil phase that surrounds relatively monomer-freewater-phase bubbles, will frequently be substantially spherical inshape. The size or "diameter" of such substantially spherical cells isthus a commonly utilized parameter for characterizing foams in generalas well as for characterizing absorbent foams of the type prepared fromthe HIPE emulsions made via the process of the present invention. Sincecells in a given sample of polymeric foam will not necessarily be ofapproximately the same size, an average cell size, i.e., average celldiameter, will often be specified.

A number of techniques are available for determining average cell sizein foams. These techniques include mercury porosimetry methods which arewell known in the art. The most useful technique, however, fordetermining cell size in foams involves simple photographic measurementof a foam sample. FIG. 1 of the drawing, for example, is aphotomicrograph of a fracture surface of a typical foam absorbentstructure prepared from a HIPE emulsion made by the process herein.Superimposed on the photomicrograph is a scale representing a dimensionof 10 microns. Such a scale can be used to determine average cell sizevia an image analysis procedure. Image analysis of photomicrographs offoam samples is, in fact, a commonly employed analytical tool which canbe used to determine average cell size of foam structures. Such atechnique is described in greater detail in Edwards et al; U.S. Pat. No.4,788,225; Issued Nov. 29, 1988. This patent is incorporated herein byreference.

For purposes of the present invention, average cell size in foams madeby polymerizing HIPE emulsions prepared herein can be used as a meansfor quantifying the amount of shear agitation imparted to the emulsifiedcontents in the dynamic mixing zone during the instant process. Inparticular, after the oil and water phase flow rates have been adjustedto provide the requisite HIPE water/oil ratio, the emulsified contentsof the dynamic mixing zone should be subjected to shear agitation whichis sufficient to eventually form a HIPE emulsion that, upon subsequentpolymerization, provides a foam having an average cell size of fromabout 5 to 100 microns. More preferably, such agitation will be thatsuitable to realize an average cell size in the subsequently formed foamof from about 10 to 90 microns. This will typically amount to shearagitation of from about 1000 to 7000 sec.⁻¹, more preferably from about1500 to 3000 sec.⁻¹.

As with the shear agitation utilized upon initial introduction of theoil and water phases into the dynamic mixing zone, shear agitation toprovide HIPE emulsions need not be constant during the process. Forexample, impeller speed can be increased or decreased during the HIPEpreparation process as desired or required to provide emulsions whichform foams having the particular desired average cell sizecharacteristics hereinbefore specified.

F) Transfer of Dynamic Mixing Zone Effluent to Static Mixing Zone

In the process of the present invention, the emulsion-containing liquidcontents of the dynamic mixing zone are continuously withdrawn from thedynamic mixing zone and introduced into a static mixing zone whereinthey are subjected to further mixing and agitation. The nature andcomposition of the effluent from the dynamic mixing zone will, ofcourse, change over time as the process herein proceeds from initialstartup to initial emulsion formation in the dynamic mixing zone to HIPEemulsion formation in the dynamic mixing zone, as the water-to-oil ratiois increased. During the initial startup procedure, the dynamic mixingzone effluent may contain little or no emulsified material at all. Afteremulsion formation begins to occur in the dynamic mixing zone, theeffluent therefrom will comprise a water-in-oil emulsion having arelatively low water-to-oil ratio, along with excess oil and water phasematerial which has not been incorporated into the emulsion. Finally,after the water-to-oil ratio of the two feed streams into the dynamicmixing zone has been increased, the dynamic mixing zone effluent willprimarily comprise a HIPE emulsion along with relatively small amountsof oil and water phase materials which have not been incorporated intothis HIPE emulsion.

Once steady state operation is achieved, the flow rate of effluent fromthe dynamic mixing zone, which becomes the feed stream to the staticmixing zone, will equal the sum of the flow rates of the water and oilphases being introduced into the dynamic mixing zone. After water andoil flow rates have been properly adjusted to provide formation of thedesired HIPE emulsion, the effluent flow rate from the dynamic mixingzone will typically rang from about 35 to 800 liters per minute forcommercial scale operations. For pilot plant scale operations, dynamicmixing zone effluent flow rates will typically range from about 0.8 to9.0 liters per minute.

The effluent continuously withdrawn from the dynamic mixing zone iscontinuously introduced into a static mixing zone for furtherprocessing. The static mixing zone provides resistance to flow of liquidmaterial through the process operations herein and thus provides backpressure for the liquid contents of the dynamic mixing zone. However,the primary purpose of the static mixing zone in the present process isto to subject the emulsified material from the dynamic mixing zone toadditional agitation and mixing in order to complete the formation ofthe desired stable HIPE emulsion.

For purposes of the present invention, the static mixing zone cancomprise any suitable containment vessel for liquid materials, whichvessel is internally configured to impart agitation or mixing to suchliquid materials as these materials flow through the vessel. A typicalstatic mixer is a spiral mixer which can comprise a tubular devicehaving an internal configuration in the form of a series of helices thatreverse direction every 180° of helical twist. Each 180° twist of theinternal helical configuration is called a flight. Typically, a staticmixer having from 12 to 32 helical flights which intersect at 90° angleswill be useful in the process herein.

In the static mixing zone, shear forces are imparted to the liquidmaterial therein simply by the effect of the internal configuration onthe static mixing device on the liquid as it flows through the deviceTypically such shear is imparted to the liquid contents of the staticmixing zone to the extent of from about 100 to 7000 sec.⁻¹, morepreferably from about 500 to 3000 sec.⁻¹.

In the static mixing zone, essentially all of the water and oil phasematerial which has not been incorporated into the emulsified material inthe dynamic mixing zone will, after HIPE water/oil ratios are achieved,be formed into a stable HIPE emulsion. Typically such a HIPE emulsionwill have a water-to-oil phase ratio which ranges from about 12:1 to100:1, more preferably from about 20:1 to 7:1. Such emulsions are stablein the sense that they will not significantly separate into their waterand oil phases, at least for a period of time sufficient to permitpolymerization of the monomeric contents of the oil phase therein.

G) Polymerizable HIPE Emulsion From the Static Mixing Zone

Emulsified material can be continuously withdrawn from the static mixingzone at a rate which approaches or equals the sum of the flow rates ofthe water and oil phase streams fed to the dynamic mixing zone. Afterthe water-to-oil ratio of the feed materials has been increased towithin the desired HIPE range and steady state conditions have beenachieved, the effluent from the static mixing zone will essentiallycomprise a stable HIPE emulsion suitable for further processing intoabsorbent foam material.

The stable HIPE emulsions having the particular composition hereinbeforespecified can be converted to useful absorbent foam materials bysubjecting these HIPE emulsion materials to suitable polymerization anddewatering conditions. In this manner, the monomeric materials presentin the external oil phase of the stable HIPE water-in-oil emulsions willpolymerize to form a solid polymeric structure in the form of a cellularfoam. The polymeric foam structure formed by polymerization of theparticular HIPE emulsions produced by the process therein are thosewhich are relatively open-celled. This means that the individual cellsof the resulting foam are, for the most part, not completely isolatedfrom each other by polymeric material filling the cell walls. Thus thecells in such substantially open-celled foam structures haveintercellular openings or "windows" which are large enough to permitready fluid transfer from one cell to the other within the foamstructure. This renders such foams especially useful as fluidabsorbents.

In substantially open-celled structures of the type which can beprepared from the HIPE emulsions produced by the instant invention, thefoam will generally have a reticulated character with the individualcells being defined by a plurality of mutually connected, threedimensionally branched webs. The strands of polymeric material whichmake up the branched webs of the open-cell foam structure can bereferred to as "struts." Open-celled foams having a typical strut-typestructure are shown by way of example in the photomicrograph set forthas FIG. 1.

Polymerization of the HIPE emulsions herein to form absorbent foams canbe brought about by placing the HIPE emulsion in a suitablepolymerization container and by subjecting the emulsion therein tocuring conditions. Such curing conditions can comprise maintenance of atemperature from about 55° to 90° C. for a period of from about 4 to 24hours. The foam materials so produced will generally also besubsequently processed to render them suitable for use as fluidabsorbents. Subsequent processing steps may include, for example, a)washing of the foam structure to remove residual water phase materialfrom the cells of the foam, b) treating the foam structure withhydrophilizing agents to render the foam internal surfaces more suitablefor absorbing hydrophilic liquids such as aqueous body fluids, c)dewatering by compression and/or heating to remove residual watertherefrom to the point such foams will be effective as absorbents foraqueous body fluids, and/or d) cutting or other shaping techniques toprovide the foam material in suitable form for incorporation intoabsorbent products. The absorbent foam materials which can be preparedfrom the HIPE emulsions prepared by the process herein are described ingreater detail in the U.S. patent application of DesMarais, Stone,Thompson, Young, LaVon, and Dyer having Ser. No. 07,743,839, (P&G CaseNo. 4451) filed Aug. 12, 1991. This application is incorporated hereinby reference.

APPARATUS

The continuous HIPE emulsion preparation process herein can be carriedout using conventional liquid processing equipment and apparatus. Atypical arrangement of such equipment and apparatus is illustrated bythe schematic flow diagram set forth as FIG. 2 of the drawing.

As shown in FIG. 2, the equipment useful for carrying out the processherein can comprise an oil phase feed tank 1 and a water phase feed tank2. Oil phase liquid is fed via oil phase feed line 3, through an oilphase feed pump 5, an oil phase heat exchanger 7 and an oil phasemetering tube 9 into a dynamic mixing vessel 11. Similarly, water phaseliquid material is fed via water phase feed line 4 through a water phasefeed pump 6, a water phase heat exchanger 8 and a water phase meteringtube 10 into the dynamic mixing vessel 11.

The dynamic mixing vessel 11 is fitted with a vent line 12. Venting ofair from the dynamic mixing vessel is controlled by a vent valve 13.Venting is required during the filling of the vessel and may be carriedout as needed to maintain an all-liquid environment in the dynamicmixing vessel 11. The metering tubes 9 and 10 are required to insuresteady stream flow of the oil and water phases into the mixer and shouldbe sized to give a pressure differential between the feed lines and themixing vessel of about 13.8 kPa (2 PSI) at the intended process flowrate.

The dynamic mixing vessel 11 is also fitted with a pin impeller 14. Thepin impeller itself comprises a shaft 15 which holds a number, e.g., 16or 17, of flights of cylindrical impeller pins 16 protruding radiallyoutwards from the impeller shaft. These flights of impeller pins arepositioned in four rows which run along a portion of the length of theimpeller shaft with the rows positioned at 90° angles around thecircumference of the impeller shaft. The rows of impeller pins areoffset along the length of the impeller shaft such that flights whichare perpendicular to each other are not in the same radial planeextending from the axis of the shaft. The impeller 14 is used to impartshear agitation to the liquid contents of the dynamic mixing vessel 11in order to form emulsified material in this dynamic mixing vessel. Suchemulsified material is withdrawn from the dynamic mixing vessel 11 viathe dynamic mixer effluent line 17, and is fed thereby into a staticmixing vessel 18.

Emulsified liquid material is subjected to further agitation or mixingin the static mixing vessel 18 and is withdrawn therefrom via a staticmixer effluent line 19. Such effluent, when in the form of a stable HIPEemulsion, can be removed via effluent line 19 and a static mixereffluent valve 20 into a suitable polymerization container 21. StableHIPE emulsion in polymerization container 21 can be subjected topolymerization conditions in order to form a desired absorbent foammaterial.

EXAMPLES

Preparation of high internal phase emulsions and their subsequentpolymerization and dewatering into absorbent foam materials areillustrated by the following examples. The procedures set forth are ingeneral carried out on a semi-plot plant scale of operation usingapparatus substantially similar to that hereinbefore described withrespect to FIG. 2 of the drawing.

EXAMPLE I

Calcium chloride (320 g.) and potassium persulfate (48 g.) are dissolvedin 32 liters of distilled water. This provides the water phase feedstream to be used in the following process for forming a HIPE emulsion.

To a monomer combination comprising styrene (420 g.), divinylbenzene(660 g.) and 2-ethylhexylacrylate (1920 g.) are added sorbitanmonooleate (450 g. as SPAN® 80) and sorbitan trioleate (150 g. as SPAN®85). After mixing, this comprises the oil phase feed stream to be usedin the following process for forming a HIPE emulsion.

At liquid temperatures in the range of 55° C. to 65° C., separatestreams of the oil phase and water phase are fed to a dynamic mixingapparatus. Thorough mixing of the combined streams in the dynamic mixingapparatus is achieved by means of a pin impeller. At this scale ofoperation, an appropriate pin impeller comprises a cylindrical shaft ofabout 18 cm in length with a diameter of about 1.9 cm. The shaft holdstwo rows of 17 and two rows of 16 cylindrical pins each having adiameter 0.5 cm extending radially outward from the central axis of theshaft to a length of 1 cm. The four rows are positioned at 90° anglesaround the circumference of the impeller shaft with the rows that areperpendicular to each other being offset along the length of the shaftas shown in the drawing. The pin impeller is mounted in a cylindricalsleeve which forms the dynamic mixing apparatus, an the pins in theimpeller have a clearance of 0.8 mm from the walls of the cylindricalsleeve. The impeller is operated at a speed of 900 revolutions perminute.

A static mixer (8 inches long by 1/4 inch outside diameter by 0.190 inchinside diameter) with the helical internal configuration hereinbeforedescribed is mounted downstream from the dynamic mixing apparats toprovide back pressure in the dynamic mixer. This helps keep the dynamicmixing apparatus comprising the cylindrical sleeve with its pin impellerfull of liquid contents. The static mixer also helps to ensureappropriate and complete formation of a HIPE emulsion from the oil andwater phases.

An emulsion having the eventually desired ratio of water to oil phasesis approached gradually. At first, flow rates are adjusted so that 3parts by weight of the water phase and 1 part by weight of the oil phaseenter the dynamic mixing apparatus with the pin impeller. The water tooil phase ratio is increased, over a period of a few minutes, until aratio of 12-13 parts water phase to 1 part oil phase is passing into thedynamic mixer, at a rate of 15 ml/sec. Gradually, the oil flow rate isdecreased so that the water phase/oil phase weight ratio is near 25:1.At this stage, the viscosity of the emulsion flowing out of the staticmixer drops. (Visually, the whitish mixture becomes more translucent atthis point.)

The flow rate of the oil phase is thereafter further decreased to thepoint where the desired water phase/oil phase weight ratio of 30-33:1 isreached. Visually, the emulsion at this stage flows from the staticmixer orifice with the consistency of a whipping cream and "sets" to aconsistency reminiscent of a creamy yogurt.

At this point, the emulsion emerging from the static mixer is ready forcuring. The emulsion is fed to a generally rectangular mold which ismade of polyethylene and which has the dimensions, 38 cm length; 25 cmwidth and 22 cm depth. Emulsion is emptied into such molds until eachmold contains approximately 20,000 ml of the emulsion to be cured.

Curing is effected by placing the emulsion-containing molds in a curingoven at a temperature of 60° C. for a period of about 16 hours. Aftercuring, the resulting solid polymerized foam material contains up to 98%water and is soft and sopping wet to the touch.

The foam material at this point may be subjected to further processingto render it suitable for use as an absorbent for aqueous body fluids.Such further processing may involve washing of the foam to removeresidual water and oil phase components, treatment of the foam withhydrophilizing agents to render its internal surfaces more hydrophilicand dewatering to provide substantially dry foam material.

When the foam material prepared as described herein is dried andsubjected to photomicrographic image analysis in the manner hereinbeforedescribed, it can be determined that such a foam material has an averagecell size of about 40 microns.

EXAMPLE II

Another HIPE emulsion (and the subsequently resulting polymeric foammaterial made therefrom) is prepared in the same general manner as setforth hereinbefore in Example I. In this example, the emulsionpreparation and polymerization procedures are carried out as in ExampleI but with the following differences in materials, concentrations andconditions:

1) An emulsifier mixture of 480 g of SPAN® 80 and 120 g of SPAN® 85 isused in the oil phase.

2) A 14 inch long×3/8 inch O.D. (35.6 cm×0.95 cm) static mixer is useddownstream from the dynamic mixing apparatus.

3) The pin impeller in the dynamic mixer is operated at a speed of 850revolutions per minute.

4) The final water to oil phase weight ratio of the HIPE emulsionproduced is 31:1.

5) A curing temperature of 66° C. is used.

After drying as in Example I, the Example II foam is subjected tophotomicrographic image analysis and is found to have an average cellsize of 37 microns.

EXAMPLE III

This example illustrates the preparation of another type of HIPEemulsion (and the subsequently resulting polymer foam material madetherefrom) falling within the scope of the present invention.

Calcium chloride (36.32 kg) and potassium persulfate (568 g) aredissolved in 378 liters of water. This provides the water phase streamto be used in a continuous process for forming a HIPE emulsion.

To a monomer combination comprising styrene (1600 g), divinylbenzene 55%technical grade (1600 g), and 2-ethylhexylacrylate (4800 g) is addedsorbitan monolaurate (960 g as SPAN® 20). After mixing, this combinationof materials is allowed to settle overnight. The supernatant iswithdrawn and used as the oil phase in a continuous process for forminga HIPE emulsion. (About 75 g of a sticky residue is discarded.)

At an aqueous phase temperature of 48°-58° C. and an oil phasetemperature of 22° C., separate streams of the oil phase and water phaseare fed to a dynamic mixing apparatus. Thorough mixing of the combinedstreams in the dynamic mixing apparatus is achieved by means of a pinimpeller. At this scale of operation, an appropriate pin impellercomprises a cylindrical shaft of about 21.6 cm in length with a diameterof about 1.9 cm. The shaft, as described in Example I, holds 4 rows ofpins, 2 rows having 17 pins and 2 rows having 16 pins, each having adiameter of 0.5 cm extending outwardly from the central axis of theshaft to a length of 1.6 cm. The pin impeller is mounted in acylindrical sleeve which forms the dynamic mixing apparatus, and thepins have a clearance of 0.8 mm from the walls of the cylindricalsleeve.

A spiral static mixer is mounted downstream from the dynamic mixingapparatus to provide back pressure in the dynamic mixer and to provideimproved incorporation of components into the emulsion that iseventually formed. Such a static mixer is 14 inches (35.6 cm) long witha 0.5 inch (1.3 cm) outside diameter. The static mixer is a TAHindustries Model 070-7821, modified by cutting off 2.4 inches (6.1 cm).

The combined mixing apparatus set-up is filled with oil phase and waterphase at a ratio of 2 parts water to 1 part oil. The dynamic mixingapparatus is vented to allow air to escape while filling the apparatuscompletely. The flow rates during filling are 1.127 g/sec oil phase and2.19 cm³ /sec water phase.

Once the apparatus set-up is filled, agitation is begun in the dynamicmixer, with the impeller turning at 1800 RPM. The flow rate of the waterphase is then steadily increased to a rate of 35.56 cm³ /sec over a timeperiod of 130 sec. The back pressure created by the dynamic and staticmixers at this point is 7.5 PSI (51.75 kPa). The impeller speed is thensteadily decreased to a speed of 1200 RPM over a period of 60 sec. Theback pressure drops to 4.5 PSI (31.05 kPa). At this point, the impellerspeed is instantly increased to 1800 RPM. The system back pressureremains constant thereafter at 4.5 PSI (31.05 kPa).

The formed emulsion flowing from the static mixer at this point iscollected in Rubbermaid Economy Cold Food Storage Boxes, Model 3500.These boxes are constructed of food grade polyethylene and have nominaldimensions of 18"×26"×9" (45.7 cm×66 cm 22.9 cm). The true insidedimensions of these boxes are 15"×23"×9" (38.1 cm×58.4 cm×22.9 cm).These boxes are pretreated with a film of a solution comprising a 20%solution of SPAN® 20 in an equal weight solvent mixture of xylene andispropanol. The solvent mixture is allowed to evaporate to leave onlythe SPAN® 20. Forty-seven liters of emulsion are collected in each box.

The emulsion-containing boxes are kept in a room maintained at 65° C.for 18 hours to bring about polymerization of the emulsion in the boxesto thereby form polymeric foam material. After curing is complete, thewet cured foam material is removed from the curing boxes.

The foam material at this point is subjected to further processing inorder to dewater the foam and leave a residual amount of the CaCl₂hydrophilizing agent incorporated within the foam structure. After suchprocessing, the foam material has a residual water content of about 5-7%by weight of polymerized material (including water of hydration) andcontains residual sorbitan monolaurate in an amount of about 11% byweight and hydrated calcium chloride in an amount of about 5% by weight(anhydrous basis).

Such a foam is of the "thin-until-wet" type meaning that such a foamwill collapse to a relatively smaller caliper upon dewatering but willreexpand in caliper when it encounters and subsequently imbides aqueousbody fluids. The thin-until-wet foam material prepared according to thisExample III will, in its expanded state, have an average cell size ofabout 15 microns.

EXAMPLE IV

This example illustrates the preparation of yet another type of HIPEemulsion (and the subsequently resulting thin-until-wet polymer foammaterial made therefrom) falling within the scope of the presentinvention.

Calcium chloride (36.62 kg.) and potassium persulfate (568 g) aredissolved in 378 liters of water. This provides the water phase streamto be used in the following process for forming a HIPE emulsion.

To a monomer combination comprising styrene (1600 g), divinylbenzene 55%technical grade (1600 g), and 2-ethylhexylacrylate (4800 g) is addedsorbitan monolaurate (480 g as SPAN® 20) and a mixture of sorbitanmonolaurate and sorbitan monopalmitate (240 g of SPAN® 20 and 240 g ofSPAN® 40) to facilitate dissolution of the SPAN® 40. After mixing, thisoil phase is allowed to settle overnight. The supernatent is withdrawnand used in the following process. About 75 g of a sticky residue isdiscarded.

At an aqueous phase temperature of 48°-58° C., and an oil phasetemperature of 22° C., separate streams of the oil phase and water phaseare fed to a dynamic mixing apparatus. Thorough mixing of the combinedstreams in the dynamic mixing apparatus is achieved by means of a pinimpeller. At this scale of operation, an appropriate pin impellercomprises a cylindrical shaft of about 21.6 cm. in length with adiameter of about 1.9 cm. The shaft holds 4 rows of pins, two rowshaving 17 pins and two rows having 16 pins, each having a diameter of0.5 cm. extending outwardly from the central axis of the shaft to alength of 1.6 cm. The pin impeller is mounted in a cylindrical sleevewhich forms the dynamic mixing apparatus, and the pins have a clearanceof 0.8 mm from the walls of the cylindrical sleeve.

A spiral static mixer (14 in. long by 1/2 in. outside diameter, a TAHIndustries model 070-821, modified by cutting off 2.4 inches) is mounteddownstream from the dynamic mixing apparatus to provide back pressure inthe dynamic mixer and provide uniformity in the emulsion.

The combined mixing apparatus is filled with oil phase and water phaseat a ratio of 2 parts water to 1 part oil, while venting the apparatusto allow air to escape while filling the apparatus completely. The flowrates during filling are 1.5 g/sec oil phase and 3.0 cc/sec water phase.

Once filled, agitation is begun, with the impeller turning at 1800 RPM.The aqueous phase is then evenly ramped up in flow to a rate of 43.50cc/sec over a time period of 40 sec. The back pressure created by thedynamic and static mixers at this point is 8.5 PSI. The impeller speedis then ramped downwardly even to a speed of 1400 PRM over a period of60 sec. The back pressure drops to 4.5 PSI. At this point the impellerspeed is instantly increased to 1800 RPM. The system back pressureremains constant thereafter at 4.5 PSI.

The formed emulsion is collected in Rubbermaid Economy Cold Food StorageBoxes, constructed of food grade polyethylene, Model 3500, nominally 18in. by 26 in. by 9 in. deep, having true inside dimensions of 15 in. by23 in. by 9 in. deep. The molds are pre-treated with a film of asolution comprising a 20% solution of SPAN® 20 in xylene (which is alsoallowed to settle overnight, and only the clear supernatant is used).The molds are pre-heated, causing xylene to evaporate to leave only theSPAN® 20. Forty-seven liters of emulsion are collected to each mold.

The filled modes are then kept in a room maintained at 65° C. for 18hours to cure the emulsion therein. After curing is complete, the wetcured foam material is removed from the curing boxes. The foam at thispoint contains about 30-40 times the weight of polymerized material(30-40X) of the residual water phase containing dissolved emulsifiers,electrolyte and initiator. The foam material is sliced with a sharpreciprocating saw blade into sheets which are 0.350 inches (0.89 cm) incaliper. These sheets are then subjected to compression in a series of 3nip rolls which gradually reduce the residual water phase content of thefoam to about 6 times (6X) the weight of the polymerized material. Atthis point, the sheets are then resaturated with a 1% CaCl₂ solution at60° c., are squeezed in a nip to a water phase content of about 10X.

The foam sheets, which now contain about 10X of what is essentially a 1%CaCl₂ solution are passed through a final nip equipped with a vacuumslot. The last nip reduces the CaCl₂ solution content to about 5 times(5X) the weight of polymer. The foam remains compressed after the finalnip at a caliper of about 0.080 in. (0.2 cm). The foam is then dried inan air circulating oven set at about 60° C. for about three hours. Suchdrying reduces the moisture content to about 5-7% by weight of thepolymerized material.

Such a foam is of the "thin-until-wet" type meaning that this foam willremain in a collapsed, relatively thin state upon dewatering but willreexpand in caliper when it encounters and subsequently imbides aqueousbody fluids. The thin-until-wet foam material prepared according to thisExample IV will, in its expanded state, have an average cell size ofabout 12 microns.

HIPE emulsions, which are prepared in accordance with the continuousprocess herein and which are especially useful for subsequentpolymerization and dewatering to form thin-until-wet absorbent foams,can be realized by selecting certain preferred processing parameters. Inparticular, thin-until-wet foam-forming HIPE emulsions can be preparedusing:

A) Oil phase emulsifiers selected from sorbitan monolaurate (e.g., SPAN®20) and combinations (e.g. in a 1:1 to 10:1 weight ratio) of sorbitanmonolaurate (e.g., SPAN® 20) and a co-emulsifier selected frompolyglycerol fatty acid esters and sorbitan monopalmitate (e.g., SPAN®40).

B) A dynamic mixing zone temperature ranging from about 25° C. to 50° C.or even 25° C. to 60° C.;

C) Shear agitation in the dynamic mixing zone which is sufficient toeventually form a high internal phase emulsion that, upon subsequentpolymerization, provides a foam having an average cell size of fromabout 5 to 30 microns; and preferably

D) Relatively higher ratios of glassy monomer to rubbery comonomer inthe oil phase such that, for example, the molar ratio of glassy monomerto rubbery comonomer ranges from about 1:2 to 1:1.

What is claimed is:
 1. A continuous process for the preparation of ahigh internal phase emulsion which is suitable for subsequentpolymerization and dewatering to thereby form an absorbent foammaterial, which process comprises:A) providing a liquid feed stream ofan oil phase comprisingi) from about 3% to 41% by weight of asubstantially water-insoluble, monofunctional glassy monomer component;ii) from about 27% to 73% by weight of a substantially water-insoluble,monofunctional rubbery comonomer component; iii) from about 8% to 30% byweight of a substantially water-insoluble, polyfunctional cross-linkingagent component, and iv) from about 2% to 33% by weight of an emulsifiercomponent which is soluble in the oil phase and which is suitable forforming a stable water-in-oil emulsion; B) providing a liquid feedstream of a water phase comprising an aqueous solution containing fromabout 0.2% to 40% by weight of water-soluble electrolyte; C)simultaneously introducing said liquid feed streams into a dynamicmixing zone at flow rates such the the initial weight ratio of waterphase to oil phase being introduced ranges from about 2:1 to 10:1; D)subjecting the combined feed streams in said dynamic mixing zone tosufficient shear agitation to at least partially form an emulsifiedmixture in said zone while maintaining steady, non-pulsating flow ratesfor the oil and water phase stream; E) steadily increasing the ratio ofwater to oil feed streams being introduced into said dynamic mixing zoneto within the range of from about 12:1 to 100:1 at a rate of increasewhich does not destroy the emulsified nature of the contents of saiddynamic mixing zone, while maintaining the emulsified contents of saiddynamic mixing zone at a temperature of from about 25° C. to 70° C., andwhile subjecting the emulsified contents of said zone to continued shearagitation which is sufficient to eventually form a high internal phaseemulsion that, upon subsequent polymerization, provides a foam having anaverage cell size of from about 5 to 100 microns; F) continuouslywithdrawing the emulsified contents of said dynamic mixing zone andcontinuously introducing said emulsified contents into a static mixingzone wherein said emulsified contents are further subjected tosufficient shear mixing to thereby completely form a stable highinternal phase emulsion having a water to oil phase weight ratio of fromabout 12:1 to 100:1; and G) continuously withdrawing said stable highinternal phase emulsion from said static mixing zone such that saidstable high internal phase emulsion can be thereafter polymerized anddewatered to form a solid absorbent foam.
 2. A process according toclaim 1 wherein:A) the glassy monomer component comprises from about 7%to 40% by weight of the oil phase; B) the rubbery comonomer componentcomprises from about 27% to 66% by weight of the oil phase; C) thecross-linking agent component comprises from about 10% to 25% by weightof the oil phase; D) the emulsifier component comprises from about 4% to25% by weight of the oil phase; and E) the water phase comprises anaqueous solution containing from about 0.05% to 20% by weight of theelectrolyte.
 3. A process according to claim 2 wherein:A) the initialweight ratio of the water phase to oil phase introduced into the dynamicmixing zone ranges from about 2.5:1 to 5:1; B) the weight ratio of thewater phase to oil phase introduced into the dynamic mixing zone isincreased to within the rage of from about 20:1 to 70:1; and C) thewater to oil phase weight ratio of the stable high internal phaseemulsion formed in the static mixing zone ranges from about 20:1 to70:1.
 4. A process according to claim 3 wherein:A) the temperature ofthe emulsified contents of the dynamic mixing zone is maintained withinthe range of from about 35° to 65° C.; and B) shear agitation isimparted to the emulsified contents of the dynamic mixing zone to theextent which is sufficient to eventually form a high internal phaseemulsion that, upon subsequent polymerization, provides a foam having anaverage cell size of from about 10 to 90 microns.
 5. A process accordingto claim 4 wherein:A) the substantially water-insoluble, monofunctionalglassy monomer component of the oil phase comprises one or morestyrene-based monomer types; B) the substantially water-insoluble,monofunctional rubbery comonomer component of the oil phase comprisescomonomer types selected from butylacrylate, 2-ethylhexylacrylate,butadiene, isoprene and combinations of these comonomer types; C) thesubstantially water-insoluble cross-linking agent component of the oilphase comprises a difunctional monomer type selected fromdivinylbenzene, divinyltolulene, diallylphthalate, one or more diacrylicacid esters of a polyol or combinations of such difunctional monomertypes; and D) the emulsifier component of the oil phase comprises anemulsifier selected from sorbitan fatty acid esters, polyglycerol fattyacid esters, polyoxyethylene fatty acids and esters and combinations ofsuch emulsifiers.
 6. A process according to claim 5 wherein:A) the molarratio of monofunctional glassy monomer component to monofunctionalrubbery comonomer component in the oil phase ranges from about 1:25 to1.5:1; and B) the cross-linking agent component is present in aconcentration ranging from about 5 to 60 mole percent, based on totalmonomers present in the oil phase.
 7. A process according to claim 5wherein:A) the water-soluble electrolyte in the water phase comprisesone or more water-soluble salts of an alkali metal or alkaline earthmetal; and B) the water phase additionally comprises from about 0.02% to0.4% by weight of a water-soluble, free radical polymerizationinitiator.
 8. A process according to claim 7 wherein shear agitation offrom about 1000 to 7000 sec.⁻¹ is imparted to the combined water and oilphase feed streams in the dynamic mixing zone.
 9. A process according toclaim 8 wherein shear agitation is imparted to the emulsified contentsof the dynamic mixing zone by means of a pin impeller.
 10. A processaccording to claim 9 wherein the shear agitation imparted to theemulsified contents of the static mixing zone ranges from about 100 to7000 sec.⁻¹.
 11. A process according to claim 9 wherein the water phaseliquid feed stream is initially fed to the dynamic mixing zone at thepilot plant scale flow rate of from about 0.04 to 2.0 liters/minute andthe oil phase liquid feed stream is initially fed to the dynamic mixingzone at the pilot plant scale flow rate of from about 0.02 to 0.2liter/minute.
 12. A process according to claim 11 wherein, after thewater to oil phase ration has been increased, the effluent from thedynamic mixing zone is withdrawn at the pilot plant scale flow rate offrom about 0.8 to 2.2 liters/minute.
 13. A process according to claim 9wherein the water phase liquid feed stream is initially fed to thedynamic mixing zone at the commercial scale flow rate of from about 25to 250 liters/minute and the oil phase liquid feed stream is initiallyfed to the dynamic mixing zone at the commercial scale flow rate of fromabout 10 to 25 liters/minute.
 14. A process according to claim 13wherein, after the water to oil phase ratio has been increased, theeffluent from the dynamic mixing zone is withdrawn at the commercialscale flow rate of from about 35 to 800 liters/minute.
 15. A continuousprocess for the preparation of a high internal phase emulsion which issuitable for subsequent polymerization and dewatering to form anabsorbent foam material, which process comprises:A) providing a liquidfeed stream of an oil phase comprisingi) from about 7% to 40% by weightof a styrene monomer component; ii) from about 27% to 66% by weight of acomonomer component selected from butylacrylate, 2-ethylhexylacrylate,isoprene, and combinations of these comonomers; iii) from about 10% to25% by weight of a divinylbenzene cross-linking agent component, and iv)from about 4% to 25% by weight of an emulsifier component selected fromsorbitan monooleate and a mixture of sorbitan monooleate and sorbitantrioleate in a monooleate to trioleate weight ratio of from about 2:1 to5:1; B) providing a liquid feed stream of a water phase comprising anaqueous solution containing from about 0.5% to 20% by weight of calciumchloride and from about 0.1% to 0.2% by weight of a water-soluble, freeradical polymerization initiator; C) simultaneously introducing saidliquid feed streams into a dynamic mixing zone at flow rates such thatthe initial weight ratio of water phase to oil phase being introducedranges from about 2.5:1 to 5:1; D) subjecting the combined feed streamsin said dynamic mixing zone to shear agitation of from about 1500 to3000 sec.⁻¹ for a period of time sufficient to at least partially forman emulsified mixture in said zone while maintaining steady,non-pulsating flow rates for the oil and water phase streams; E)steadily increasing the ratio of water to oil feed streams beingintroduced into said dynamic mixing zone to within the range of fromabout 20:1 to 70:1 at a rate of increase which does not destroy theemulsified nature of the contents of said dynamic mixing zone, whilemaintaining the emulsified contents of said dynamic mixing zone at atemperature of from 35° C. to 65° C., and while subjecting theemulsified contents of said zone to continued shear agitation of fromabout 1500 to 3000 sec.⁻¹ for a period of time which is sufficient toform a high internal phase emulsion that, upon subsequentpolymerization, provides a foam having an average cell size of fromabout 10 to 90 microns; F) continuously withdrawing the emulsifiedcontents of said dynamic mixing zone and continuously introducing saidemulsified contents into a static mixing zone wherein said emulsifiedcontents are further subjected to sufficient shear mixing to therebycompletely form a stable high internal phase emulsion having a water tooil phase weight ratio of from about 20:1 to 70:1; and G) continuouslywithdrawing said stable high internal phase emulsion from said staticmixing zone such that said stable high internal phase emulsion can bethereafter polymerized and dewatered to form a solid absorbent foam.